Curable resins are suitable for a variety of applications. For example, curable resins have been used as coatings, sealants and adhesives, and in producing molded articles. The manufacturing facilities that utilize conventional curable resins in their operations are typically expensive to maintain due to the nature of these resins. Conventional curable resins are typically solvent-based and contain volatile organic components (VOCs). VOCs favorably affect the viscosity of curable resins, and often VOCs are added so as to make the curable resin composition sprayable. This is a particularly desired quality for coating applications (e.g. furniture, automobile, ship and other transportation vehicle coating applications). However, during the application and cure of these conventional resin compositions, substantial amounts of VOCs are released into the atmosphere. This is a serious problem because VOCs are toxic, flammable, explosive, smog-producing and noxious. As a result, elaborate and expensive fire and explosion-prevention measures, worker protection measures and pollution control equipment are required during the use of such conventional resins.
An additional cost is generated when aged and damaged cured resins are stripped and replaced with fresh curable resins. In the past, methylene chloride based chemical strippers were used to remove cured resins, but the use of these strippers has now been prohibited by the EPA. Alternatively, a number of other solvent-based stripping agents have been used. However, such solvent-based stripping agents require expensive pollution control equipment. In addition, the disposal of the removed resin and the solvent-based stripping agent further impacts the environment and increases costs. Presently, new, “environmentally compliant” chemical strippers are used in combination with non-chemical methods, such as sanding.
A number of resin manufacturers are currently developing low/no VOC curable resin formulations including, high solids, blocked isocyanate polyurethanes and aqueous-based polyurethane dispersions. These emerging resins possess certain advantageous attributes, but not without specific drawbacks. High solids formulations use low VOCs and provide equivalent performance to low solids urethanes, but cannot be applied using spraying techniques. Blocked isocyanate polyurethanes also use low VOCs and have low toxicity levels and good properties. These materials however, must be cured at elevated temperatures around 150 to 200° C., which is not practical in most applications, for example in coating automobiles. Aqueous based polyurethanes use no VOCs, are nontoxic and easy to apply via spraying. To date, however, these polyurethanes have not provided the properties required to meet military specifications, such as adhesion and moisture-resistance, desired for high performance applications.
Polyesters have emerged as promising candidates for inclusion as components in advanced aircraft coatings. Polyesters are polymers formed by the reaction of aliphatic or aromatic polyol monomers with aromatic or aliphatic diacids in the presence of catalyst, usually, metallic acetates, such as calcium acetate. Polyesters can provide a combination of useful properties including: clarity, transparency, absence of color, flexibility, excellent adhesion to most substrates, abrasion-resistance, water-resistance, fuel and oil-resistance, UV-resistance, weather-resistance, variable hardness (from elastomer to tough plastic), moderate cost, low temperature flexibility and availability as variable molecular weight, ester-endcapped, difunctional prepolymers.
Polyester polymers cannot be used as curable resins themselves, however, since their functionality is not conducive to the formation of hard, cross-linked structures. To be used advantageously as advanced coatings, polyester polymers must first be functionalized with reactive groups, such as hydroxyl groups, and then cured by chemical reaction with other comonomers, such as polyurethane diisocyanate monomers, into hard, tough polymer resins that can be used as protective coatings. Hydroxy-functional, polyester-based reactive prepolymers are frequently selected for use as components in polyurethane coatings over polyether and other type, polymers since they typically impart superior solvent-resistance, mechanical toughness and clarity to the coating and do not yellow upon exposure to intense UV light.
The major problems associated with the use of polyester-derived polyurethanes have involved the requirement for addition of substantial quantities of flammable, toxic solvents to polyurethane coating formulations to lower viscosity sufficiently for spray-painting operations. Another problem involved the use of toxic, irritating and volatile diisocyanate monomer components as major components in the polyurethane coating. The vapors that evolve from polyurethane operations may cause serious injury to workers breathing the fumes if they do not wear protective equipment.
A number of polyester-polyurethane coating manufacturers have been actively pursuing the development of low VOC and no VOC polyurethane curable resin formulations. To date however, none of these resins have exhibited the required processability or properties for use in e.g., automotive coatings.
Accordingly, what is needed are high performance curable resin compositions that contain no or substantially no VOCs and that have properties suitable for use in e.g., adhesive, coating and sealing applications, and which can be removed by use of an agent that readily, effectively and safely removes such curable resin compositions upon demand. In the case of coatings, it is desired that such compositions cure to a hard, cross-linked structure.